1. Field of the Invention
The invention relates generally to apparata and methods for measuring thermal effusivity distribution of micro-scale region by the thermoreflectance technique, and more particularly, for calculating the thermophysical property of specimen in micro-scale by focusing a heating laser beam and a probe laser beam at the same point on the specimen and detecting the reflection of the probe laser beam.
2. Description of the Related Art
Although the laser flash method is well established for thermal diffusivity measurements, the method requires the disk-shaped specimen have a thickness of more than 1 mm and typically 10 mm in diameter. The thermal diffusivity value measured with this method is the averaged value over the entire specimen.
However, the laser flash method is unsuitable to measure thermophysical property distribution of a smaller region, namely micro-scale (e.g., specimens having dimensions on the order of micrometers or smaller), which is especially important in modern microelectronics industries, especially in microelectronic devices, and large storage media that are increasingly complex and miniaturized for higher integration and quality.
Although thermophysical property values of small areas are needed for heat transport simulation of micro devices, in general, measurement of such values is difficult compared to measurement for bulk materials.
In the related field, thermal diffusivity measurement systems with a picosecond thermoreflectance technique have been developed to measure thermal diffusivity of submicrometer thin films. However, these systems are not suitable for thermal property distribution measurements, since the diameter of the measured area is larger than 50 micrometers and the technique takes 30 minutes to measure the value at one point.
Primary objectives of this invention are to solve the problems of these conventional techniques and to provide methods and apparata for thermal diffusivity measurement, which are capable of measuring the thermophysical property distribution of a specimen""s micro-scale surface with high spatial resolution.
For achieving these objectives, one embodiment of the invention provides an apparatus for micro-scale thermophysical property measurements including a heating laser system which produces a heating laser beam that heats the surface of the specimen, a modulator which sinusoidally modulates the intensity of the heating laser beam, a probe laser system, which produces a probe laser beam that is impinged on the heated surface of the specimen, a microscopic optics that focuses both beams at the same point on the surface, a photo detector for detecting the reflection of the probe laser beam and determining the temperature change of the surface based on the temperature dependence of reflectivity at the surface, and a computer which calculates the local thermophysical property of the specimen based on the reflection detected above.
In an alternate embodiment of the invention, the apparatus calculates the thermophysical property value from the phase lag of the reflected probe beam from the heating beam.
In an alternate embodiment of the invention, the apparatus calculates the thermophysical property value from the ratio of the relative intensity difference of the reflected probe beam to that of the heating beam.
In an alternate embodiment of the invention, the apparatus coats a metallic thin film on the specimen""s surface.
In yet another alternate embodiment of the present invention, the apparatus measures the two-dimensional distribution of the thermophysical property of the specimen by translating the specimen set on an X-Y stage relative to the microscopic optics.
In one embodiment of the invention, a specimen""s surface is heated with a modulated heating laser beam whose spot size is only several micrometers. By focusing the heating laser beam and a probe laser beam at the same point, the phase of the surface temperature delays from the phase of the modulated heating laser beam because of the heat diffusion into the specimen. Since the reflection of the probe laser beam proportionally changes with specimen""s surface temperature, the phase lag depends on the thermophysical property of the specimen. Therefore, the amplitude and the phase lag of the temperature change of the specimen""s surfaced could be measured by lock-in amplifying the signal of reflection of the probe laser beam, that is detected by the photo-detector, with the reference signal. In one embodiment, the intensity of the probe laser beam proportionally changes with the surface temperature.
Since the phase lag of the surface temperature change is smaller for larger absorption coefficient, xcex1, and thermal diffusivity, k, xcex12k could be calculated from the phase lag induced by the intensity change of the reflection of probe laser beam to the heating laser beam. In one embodiment, the intensity change of the probe laser beam is proportional to the surface temperature change. Since the temperature change is larger for larger absorption coefficient, xcex1, and thermal diffusivity, k, xcex12k could be calculated from the ratio of the intensity change of probe laser beam to that of the heating laser beam. In another embodiment, even if a specimen has small absorption coefficient to the heating laser beam and small reflectivity change to the probe laser beam, the thermoreflectance technique is applicable to the specimen coating with a metal thin film of large thermoreflectance effect.
According to another embodiment of the invention, C/bs could be calculated from the phase lag of the reflected probe laser beam to the heating laser beam since the phase lag is small when the specimen has small heat capacitance, C, and large thermal effusivity, bs.
According to yet another embodiment of the invention, C/bs could be calculated from the relative intensity of the probe laser beam to that of the heating laser beam.
According to still another embodiment of the invention, two-dimensional distribution of local thermophysical property could be calculated by measuring the phase lag and the relative intensity while translating the specimen set on the X-Y stage in two-dimensions.